Surfactants lower the surface tension of a liquid thereby imparting improved surface effects such as spreading, wettability, penetrability, foam inhibition and dispersibility. These improved surface effects are advantageous in many industrial applications including aqueous coatings such as inks, paints, varnishes, and the like.
Equilibrium surface tension refers to the surface tension measured after the liquid and added surfactant have reached equilibrium. Liquids with poor (high) equilibrium surface tension may initially spread smoothly and evenly across a surface but after some time will “de-wet” resulting in undesirable surface defects. This occurs because the passage of time allows the liquid and added surfactant to reach an equilibrium surface tension which is undesirably high thereby causing a liquid which was initially smooth and evenly spread to “crawl back” or “retract” from the surface thereby creating an uneven and rippled spreading. Poor equilibrium surface tension is particularly detrimental in paints which are expected to dry as a smooth and even coating.
Two common surfactants, used to lower equilibrium surface tension, are fluorosurfactants and siloxane surfactants. In general, fluorosurfactants impart significantly lower equilibrium surface tensions compared with siloxane surfactants. However, because fluorosurfactants are typically higher in cost compared with siloxane surfactants, fluorosurfactants are often mixed with less expensive siloxane surfactants to produce surfactant compositions which adequately lower equilibrium surface tension at lower cost. For example, U.S. Pat. No. 5,852,075 discloses a surfactant composition comprising a fluorosurfactant and a siloxane surfactant. Although it would be desirable from a cost perspective to use lower amounts of fluorosurfactant, U.S. Pat. No. 5,852,075, as well as many other relevant documents, lacks any specific teaching of surfactant compositions comprising a fluorosurfactant and a siloxane surfactant wherein the fluorosurfactant is present in an amount less than 22 weight percent.
A teaching of fluorosurfactant in lower amounts is absent perhaps because of an expectation that surfactant compositions comprising amounts lower that 22 weight percent of fluorosurfactant would not adequately lower equilibrium surface tension. The expectation is that decreasing the amount of fluorosurfactant mixed with siloxane surfactant would lead to a proportionate decrease in the resulting surfactant composition's ability to lower equilibrium surface tension. Accordingly, it would be advantageous to identify specific fluorosurfactants and siloxane surfactants that combine to produce surfactant compositions which adequately lower equilibrium surface tension wherein the amount of fluorosurfactant in the surfactant composition is less than 22 weight percent.